The demand for head-up display (HUD) systems on airplanes is increasing as more pilots become acquainted with the advantages of the systems. Briefly described, a HUD system typically includes a viewing element, called a combiner, that is placed in a pilot's forward field of view. The combiner is substantially transparent but is treated to be reflective to certain wavelengths of light. When symbolic information is projected onto the combiner from a relay lens using those wavelengths, the combiner refocuses the information at optical infinity and the information is overlaid on the pilot's forward field of view. A HUD system may therefore be advantageously used to provide information to the pilot, especially when the pilot needs to maintain eye contact with the forward field of view. Such situations include all phases of flight, but use of a HUD during take-off, landing, and taxi operations offer the greatest benefits.
Another technology that provides added situational awareness to an aircraft pilot is the use of imaging sensors. A sensor such as a camera can provide real-time images of scenes of interest to the pilot. Imaging sensors are especially useful when the sensors are configured to sense non-visible radiation wavelengths. For example, runway lights may be detected in the near infra-red wavelength range even if inclement weather partially obscures lights from the pilot's view in the visible wavelength range.
There has been some interest in displaying images from an imaging sensor using a HUD such that the displayed image is conformal with, or overlays, a pilot's view through the windshield. Such a combination of situational technologies (HUD plus imaging sensor), known as an enhanced vision system or EVS systems, would further assist a pilot in guidance and navigation. However, several challenges must be overcome to effectively provide such an integrated EVS system. For example, there is some controversy as to which sensor technology provides the most useful or relevant information to a pilot. The answer to this controversy appears to be that because of the wide variety of useful information that could be sensed, there is no single “best” sensor; instead, a combination of sensors may provide the best overall capability. Of particular interest are long or mid-wavelength infrared (IR) sensors, which can be used to detect thermal emissions and thereby provide compelling thermal images of a runway environment. However, long and mid-wavelength IR cannot directly detect landing lights. Also of interest are short-wavelength IR, or near IR sensors, which are useful in detecting approach lights as well as runway centerline and edge lights in an airstrip touchdown zone.
One of the major challenges associated with the image sensing portion of known EVS systems is finding a physical location for the sensor or sensors. The radome of the aircraft is often identified as the best location because of the sensor view from the radome, because an IR transmitting window can be integrated into the radome, and because there is a space envelope to house the sensor within the radome. Known IR transmitting windows allow both long and mid-wavelength IR and near IR to pass to the sensor.
A problem with installing the imaging sensor in the radome is that the sensor is physically located away from the pilot's eyes. The difference between the sensor field-of-view and the pilot's field-of-view introduces parallax errors between the sensor's view and the pilot's view. This can cause a significant issue when the short-wavelength IR sensor is placed in the radome. In such a situation there is a difference between the position of the landing lights directly seen by a pilot through the HUD combiner, and the same landing lights sensed by the short-wave IR sensor and displayed on the HUD, thereby causing a visual misregistration of the landing lights, which under certain circumstances could cause a loss of confidence in the EVS system. The misregistration between the pilot's view of the real world and the sensor view is a function of the physical spacing between the sensor and the pilot, as well as the distance to the object being viewed. Parallax errors are largest for objects located directly in front of the aircraft, such as runway centerline lights and taxi edge lights.
One method of overcoming parallax error is disclosed in U.S. patent application Ser. No. 10/454,015, entitled “Integrated Enhanced Vision System,” invented by Robert B. Wood, and incorporated herein by reference in its entirety. Said patent application discloses a short-wave IR sensor mounted on the HUD combiner and configured to look out through the aircraft windshield. The short-wave IR sensor detects IR wavelengths that pass through the windshield. This method essentially eliminates parallax error between the pilot's view and the detected short-wave IR because of the close proximity of the sensor to the pilot's eyes. However, medium to long-wavelength IR sensors could not be similarly positioned because the aircraft windshield would likely block such IR wavelengths. Furthermore, completely compensating for parallax errors is also difficult and expensive and adds display latency.
It is therefore an object of the invention to provide an enhanced vision system for an aircraft that minimizes parallax errors between what an imaging sensor detects and what an aircraft pilot sees through the windshield of the aircraft.
It is also an object of the invention to provide an enhanced vision system that minimizes or eliminates any re-certification requirements of the aircraft or of other aircraft systems.
It is further an object of the invention to provide an enhanced vision system that eliminates expensive modifications to the aircraft.
A feature of the invention is a near-infrared sensor physically integrated into a HUD system.
An advantage of the invention is the essential elimination of parallax errors between readily recognizable real-world objects detected by the sensor and the same object directly viewed by the pilot through the combiner.